In 2011, in Mayo Foundation for Medical Education and Research v. United States, the Supreme Court held that general authority Treasury regulations adopted using notice-and-comment rulemaking carry the force of law and thus are eligible for Chevron deference. In the wake of Mayo, courts and scholars are now struggling with its implications for whether temporary Treasury regulations and IRB guidance documents (revenue rulings, revenue procedures, and notices) that lack notice and comment but are enforceable through civil penalties are likewise eligible for Chevron deference and, relatedly, whether these formats are in fact subject to APA notice-and-comment rulemaking requirements. Currently prevailing judicial tests for evaluating these questions do not offer clear or easy answers for the tax context. Ultimately, both questions turn on whether the agency actions in question carry “the force of law.” The purpose of this Article is to take a step back from existing doctrinal standards and to sort through the basic administrative law principles and Supreme Court precedents that drive those standards in an effort to develop a coherent approach to Treasury and IRS rulemaking and judicial review thereof.